The invention relates to a support structure, or chuck, for a wafer table allowing accurate positioning of the wafer table on the support structure under changing thermal conditions. Such support structures are known in the art, for instance from U.S. Pat. No. 4,758,091 describing an apparatus for positioning a work piece, the apparatus comprising support means for supporting said work piece, wherein said support means includes one or more reference planes for establishing a desired position of said work piece along at least two orthogonal coordinate directions, wherein the reference planes are comprised of a reflective surface mounted on said support means and perpendicular to a corresponding coordinate direction, substrate means for introducing translational movement to said work piece, and coupling means for coupling said support means to said substrate means such that thermal expansion of said substrate means is isolated in said support means. According to the US patent the known support means preferably comprise a material having low thermal expansion coefficient, such as Zerodur™, for fixing the spatial relationship between the reference surface and the work piece.
A drawback of the known apparatus is that low thermal expansion materials are often costly and difficult to machine.
It is an object of the present invention to provide a support structure for a wafer table which is easier and less costly to manufacture.